1. Field of the Invention
This invention pertains generally to gas separation processes. More specifically, the present invention uses voltage sensing techniques to optimize performance of an electrostatic precipitator. In a most specific manifestation, a novel method is provided which maximizes the product of the electric field at the collector plate and the charge of the particles being collected.
2. Description of Related Art
Industries as diverse as mills, pharmaceutical or chemical, food processing, and cement kilns must separate contaminants or particulates from an air or gaseous stream. The gases may be a product of combustion, such as present in an exhaust stack, but may also represent other gas streams and may contain such diverse materials as liquid particulates, smoke or dust from various sources, and the like. Separators that must process relatively large volumes of gas are common in power generating facilities and factories.
The techniques used for purification of gas streams have been diverse, including such techniques as filtration, washing, flocculation, centrifugation, and electrostatic precipitation. Each technique has heretofore been associated with certain advantages and disadvantages. These features and limitations have dictated application.
Electrostatic precipitators have demonstrated exceptional benefit for contaminants including fly ash, while avoiding the limitations of other processes. For example, unlike centrifugation, electrostatic precipitators tend to be highly effective at removing particulates of very minute size from a gas stream. Unlike filtration, the process provides little if any flow restriction, and yet substantial quantities of contaminants may be removed from the gas stream.
When contaminants pass through an electrostatic precipitator, they first pass near precipitator electrodes, which transfer an electrostatic charge to the contaminants. Once charged, the contaminants will be directed by electrostatic force towards oppositely charged collecting electrodes. The collecting electrodes are frequently in the form of plates having large surface area and relatively small gap between collector plates. The dimensions of the plates and the inter-electrode spacing is a function of the composition of the gas stream, electrode potential, particulate size of contaminants, anticipated gas breakdown potential, and similar known factors. The selection of dimension and voltage will be made with the goal of gas stream purification in mind, and in gas streams where very fine particulate matter is to be removed, such as with fly ash, relatively high voltage potentials and larger plates may be provided. The proper transfer of charge to the particulates and the subsequent electrostatic attraction to collector plates is vital for proper operation.
Unfortunately, as the particulates precipitate onto the collector plates, a precipitate layer accumulates and increases in thickness. In the situation in which the particulate matter comprises high resistivity material, the large voltage drop across the high resistivity precipitate layer reduces the voltage differential between the cathode wires and the surface of the precipitate layer, in turn reducing particulate charging and collection. Moreover, the precipitate layer has the characteristics of both resistance and capacitance. When a high electric field gradient is created within the precipitate layer, this may lead to a back-corona discharge or sparking. High resistivity precipitate layers can exhibit back-corona phenomena in which ions are actually emitted from the precipitate layer toward the cathode wires, thereby additionally reducing particle charging and collection. Even though the precipitate layer may be periodically removed by means of rapping or the like, there is still an efficiency reduction concomitant with the formation of this highly-resistive layer.
A problem remains in the powering of electrostatic precipitators to provide control of the precipitator to prevent back-corona or sparking voltages, while at the same time maintaining peak particulate collection efficiency. Accordingly, efficient but yet effective and economical ways of energizing precipitators are highly desirable, particularly for the collection of particulates exhibiting medium to high resistivity. Such dusts are, for example, created in the burning of low sulfur coal used by the electric utility industry.
Newer designs for electrostatic precipitator power supplies operate at frequencies between 1500 and 30,000 Hertz and produce a nearly pure DC voltage and current input to the electrostatic precipitator. This method of energization improves the performance of a precipitator collecting low resistivity particulate such as may be produced from a high-sulfur coal fired utility or the particulate and mist encountered in a wet electrostatic precipitator. However, for the moderate to high resistivity applications identified herein above, such as in low-sulfur coal fired utilities, pure DC energization is not always optimal. In the industry, power supplies that are capable of intermittent energization are adjusted using a trial and error method that uses a secondary electrostatic precipitator performance indicator such as opacity of the gas stream as the measure of performance. These procedures do not necessarily produce a true state of optimum performance because opacity is not a definitive or sensitive measure of the performance of an individual electric field. What is desired then is a method or apparatus to overcome these limitations of the present electrostatic precipitator power supplies when applied to moderate to high resistivity particulate.